Chief Justice John G. Roberts Jr. called on President Obama and the Senate on Friday to solve what he called "the persistent problem of judicial vacancies."

"We do not comment on the merits of individual nominees," Chief Justice Roberts wrote on Friday. "That is as it should be. The judiciary must respect the constitutional prerogatives of the president and Congress in the same way that the judiciary expects respect for its constitutional role."

But he identified what he called a systemic problem.

"Each political party has found it easy to turn on a dime from decrying to defending the blocking of judicial nominations, depending on their changing political fortunes," he said.

The upshot, he said, was "acute difficulties for some judicial districts."